Three-phase motors are used in various applications. The motors are typically energized with a power source in one of two forms. In one form, the input signal is a sine wave with each of the three motor leads being driven with a voltage or current sine wave that has a 120 degree phase difference between each lead. In the other form, “trapezoidal commutation” is used to provide a controlled current that drives only two of the three motor leads at any point in time. Sine waves are typically used to drive larger, higher performance motors. Lower cost trapezoidal commutation is used for small motors.
Three-phase motors driven by sine waves are controlled by a motor drive that regulates the phase and amplitude relationship of the three sine waves being supplied to the motor. Most three-phase motor drives include circuitry to measure current in the motor leads. These current measurements are typically used in a feedback loop to adjust motor lead voltages and obtain desired current values. These measurements, however, may be used simply to detect excessive currents, which imply a fault condition.
Three-phase motor currents are conventionally sensed by one of three methods. In one method, which is used for motors driven by trapezoidal commutation only, a single sense resistor is used to measure current in a supply leg common to all three H-Bridge drivers. In another method, three sense resistors, one in the supply leg of each H-Bridge driver, are monitored to measure current to the motor. The third method uses three current sense elements with each one being in series with one of the motor leads. Alternatively, this method may be performed with two sense resistors. Each one of the sense resistors is connected in series with a motor lead and the third current is calculated as the negative sum of the currents in the other two leads.
The first technique may be implemented at a cost lower than the other two, but it suffers from two limitations. As already noted, it is only utilized with trapezoidal drives, which typically drive small three-phase motors, because it is considered too inefficient for larger motors. Secondly, this approach requires the sense resistors to be directly in the current path. Consequently, current transformers, which are used to sense current in larger motors, may not be used with this method because the DC component of the current presents flux issues.
While the other two methods are suitable for use with larger three-phase motors, they are generally more expensive to implement. Additionally, increased switching losses may arise from the typical sinusoidal drives used with these methods. In conventional sinusoidal drives, all three motor leads are driven at all times. The relatively continuous switching causes more losses and is therefore less efficient than methods that selectively drive fewer than all of the leads. Thus, larger three-phase motors driven by conventional sinusoidal drives need more economical current sensing methods and more efficient driving methods.